Method for preventing high-temperature blistering of copper coatings electro-deposited on copper substrates

ABSTRACT

A method for preparing the surface of copper substrates by anodically dissolving the surface, for a time and depth sufficient to substantially decrease higher local oxygen content of the surface and thereby improve the adhesion of electrodeposited copper coatings. More specifically, the surface pretreatment provides a base for an electrodeposited copper coating with enhanced resistance to blistering and peeling at temperatures in excess of 700* F.

United States Patent Hamilton et al.

[ June 20, 1972 [54] METHOD FOR PREVENTING HIGH- TEMPERATURE BLISTERING OF COPPER COATINGS ELECTRO- DEPOSITED ON COPPER SUBSTRATES l 72 I Inventors: Colin B. Hamilton, Monroeville; Edward J. Oles, Jr., Pittsburgh, both of Pa.

[73] Assignee: United States Steel Corporation [22] Filed: Sept. 11, 1970 [21] Appl.No.: 71,559

[52] U.S. Cl. ..204/32 R, 156/18, 204/16 [51] Int. Cl. ..C23b 3/02 [58] FieldoiSearch ..156/18; 204/16, 32R, 141

[56] References Cited UNITED STATES PATENTS 1,995,200 3/1935 Cubitt et al. l 56/18 2,461,228 2/1949 Miles ..204/32 R Primaly Examiner-John H; Mack Assistant Examiner-W. 1. Solomon Att0meyArthur Greif ABSTRACT 8 Claims, 2 Drawing figures P'A'TENTEDJum 1972 500 500 INVENTORS COL //V B. HAM/L 7'0/V 8 EDWARD J. 0L JR y Ar torjey 200 300 Bulk 0 Content (ppm) METHOD FOR PREVENTING HIGH-TEMPERATURE BLISTERING OF COPPER COATINGS ELECTRO- DEPOSITED ON COPPER SUBSTRATES Copper has found extensive use as a mold material in metal casting processes because of its excellent heat transfer properties, however, its poor abrasion and wear resistance in considerable reconditioning expense to insure high-quality castings. Electrodeposition of copper is an excellent method for reconditioning such worn mold surfaces, where up to 0.1 inches of plated copper may be required to refinish the surface of the mold. These deposits must withstand the elevated temperature of the casting operation which may range from 500 to about 1,300 F. Experimental work leading to this invention has confirmed reports (e.g. Spiro, Electroforming, Robert Draper Ltd., England, 1968, p. 28-30) that copper coatings deposited from acid-copper electrolytes, blister and peel at temperatures in excess of 500 F. Typical treatment of copper substrates prior to plating involve conditioning by machining, solvent degreasing, soak cleaning, mechanical scrubbing, pickling, and electropolishing. Various combinations of these treatments may be employed, and in general, most preplating sequences are satisfactory in producing an adherent copper coating at room temperature, i.e. a coating which is adherent in an 180 bend test. However, after heating such coatings to temperatures in excess of 500 F., blisters develop and the coatings can be easily peeled from the substrate. Even coatings which are free of blisters after heating to 500 F. may still be peeled from the basis metal, indicating that adhesion has deteriorated, even though blisters have not formed.

The pretreatment of this invention provides a base or substrate for a coating with enhanced adhesion and markedly decreased tendency to blistering at elevated temperatures. The pretreatment involves anodic dissolution (either electrochemical or chemical) of the base metal prior to plating, so that high surface oxygen concentrations leading to blister formation are substantially eliminated. Surface oxygen content, for purposes of this invention, is determined by a warm extraction method," in which pure H is passed over the surface of the metal maintained at a temperature of about 300 C. The surface oxides and the adsorbed oxygen is thereby chemically reduced, forming H O. The amount of oxygen is then calculated from a knowledge of the H passed into, and H vapor formed in the system.

The minimum depth of removal is principally dependent on three factors: l the bulk oxygen content of the base metal as measured by standard vacuum fusion analysis: (2) the severity of the machining and grinding operations employed in conditioning of the base metal surface; and (3) the maximum temperature which will be encountered by the plated article.

The relationship of these factors as well as other objects and aspects of the invention will be better understood by reference to the following description, read in conjunction with the appended claims and figures, in which:

F IG. 1 is a graphical representation of the effect of depth of anodic dissolution on surface oxygen content for two substrates of varying bulk oxygen content, and

H6. 2 represents the minimum range of dissolution required to provide resistance to peeling and blistering at temperatures of 1,100 F and l,300 F.

Experiments leading to this invention have revealed that the blistering tendency of copper coatings is primarily associated with higher localized oxygen content of the surface layers of copper articles. Removal of such oxygen-contaminated surface layers, which may be accomplished by either chemical or electro-chemical dissolution, eliminates this high localized ox ygen concentration and the consequent lack of adhesion of plated coatings.

As stated hereinabove, the amount of metal that must be removed is primarily dependent on three factors:

1. The severity of the machining or grinding operations which have been employed. Because of the difficulty in quantification, for purposes of better defining the invention, the severity of such operations has been classified as either normal or heavy on the basis of the depth of the resultant disturbed metal layer, as determined by metallographic examination. Thus a machining operation (milling, fly-cutting, grinding, etc.) which by reason of pressure and cutting speed resulted in a depth of grain elongation (or other type of grain distortion) of less than 2.0 mils would be considered "normal, while a heavy" operation would produce a disturbed metal layer in excess of 2.0 mils. The more severe mechanical treatment producing a greater heat build up, thereby resulting in a larger amount of local oxygen inclusions. However, even for base articles which have received identical mechanical treatments, the minimum depth of anodic dissolution will also be dependent on,

2. The bulk oxygen content of the base material. As shown in FIG. 1, two substrates of different bulk oxygen contents, received identical prior mechanical conditioning treatment resulting in a disturbed metal layer less than 2.0 mils in depth. At the outset, the oxygen contents of the surfaces due to such treatment were therefore substantially identical. However, after anodic dissolution to a depth of less than 1 mil, the surface oxygen content at the surface of low bulk oxygen substrate (25 ppm) was markedly lower than that of high bulk oxygen substrate (470 ppm). It may therefore be seen, that the process of this invention is more efficient for substrates which originally possess low oxygen contents, preferably those with oxygen contents less than ppm. The bulk oxygen content of the base metal is also important in another regard. Since oxygen, unifomily dispersed throughout the base metal is not eliminated by this dissolution process and is still available for reaction, maximum coating performance will only be obtained by using a material which is already low in oxygen content, in conjunction with the pretreatment of this invention.

3. The maximum temperature to be encountered by the electroplated article. The higher the service temperature for which resistance to blistering and peeling is desired, the greater must be the depth of removal. Since the minimum depth of removal for any given service temperature is not only dependent on the bulk oxygen content of the base metal but is also dependent on the severity of the preceding grinding and machining operation, the depth of removal cannot be exactly specified for every possible case. However, a number of tests have indicated that a practical minimum of removal may be approximated for the scope of normal machining operations. This minimum range is represented in H6. 2, for service temperatures. of l,l00 F and l,300 F. A depth of metal removal greater than the indicated minimum (shaded area) could, of course, be employed to insure proper cleanliness. However, it would generally be less desirable for economic reasons, since excess power and electrolyte would be consumed both for the dissolution treatment and the subsequent plating operation.

The pronounced role of bulk oxygen content may also be seen by reference to FIG. 2. If a maximum service temperature of 1,l00 or below were desired, it would only be necessary to remove about 0.12 mils of surface for a low bulk oxygen (e.g. 25 ppm) base metal whereas a removal of at least about 1.1 mils would be required for a high bulk oxygen (e.g. 450 ppm) base metal (assuming the prior machining operations were not severe). If resistance to peeling were desired at 1,300 E, the minimum of removal would be about 0.15 and l .3 mils respectively.

The effectiveness of the pretreatment of this invention may be seen by recourse to the following specific examples. Copper panels were either milled or fly-cut, thereby producing a depth of disturbed metal, as determined by. visual examination of the grain distortion, which was less than 1.5 mils. These panels containing varying amounts of bulk oxygen were swabbed with benzene to remove cutting oils, scrubbed with an abrasive alkaline cleanser and thoroughly rinsed with water (e.g. conventional pretreatment). In each case, a duplicate panel of the same substrate material was similarly prepared, but was additionally treated by being placed in an electrolyte containing 300 g/l CrO plus 3 g/l H 50 and maintained at F. These duplicate panels were anodically dissolved at a current density of 50 A/ft for a dissolution time of 30 minutes, thereby providing a removal of 1.8 mils of metal surface (and thus insuring that a sufficient amount of metal had been dissolved). After thorough rinsing in water, all the panels oxygen contents within the contemplated range (less than 600 ppm), one may insure the enhancement of resistance to blistering and peeling by the dissolution to a depth of at least about 1.0 mil of metal.

Low Oxygen Containing Copper Substrate (25 p.p.m.)

1 Conventional es. 0.06 s... Yes. Yes... Yes. 0.12 No Yes..... 1es..... Yes 0.18 No No... No...... .\'o. 0.60 No No Now... No.

were plated with copper for about l5 hours duration to deposit a coating approximately 30 mils thick. Each panel was then cut into several test coupons, which were used to determine the adhesion characteristics of the coating at several temperatures. After each coupon was maintained at temperature for 30 minutes, it was inspected for blisters; coating adhesion was then evaluated by subjecting the coupon to a 180 bend test. The results are reported in Table I.

The metal dissolution in accord with this invention may be accomplished by a variety of methods known to the art. For example, in addition to the electrolyte dissolution of the above examples, chemical dissolution has been found to be quite satisfactory. Thus a removal rate of about 0.06 mil/minute has been achieved in a solution maintained at 80 F. and composed of 500 g/l CrO and 50 g/l H 80 No matter what type of dissolution is employed, to achieve optimum results the TABLE I Copper Oxygen 700 F. 1,000 F. 1,300 F. 1,400 F. 1,500 F. 1,900 F. basis content inetnl (p.p.m.) Pretreatment Blister Peel Blister Peel Blister Peel Blister Peel Blister Peel Blister Peel Conventional No..." No.... No..... No.... Yes.... Yes lMetulreniovnl (invent)... No..... No.... No..... No.... No..... No No..... No No B iConvontionnl No.. No. No. No.... Yes.... Yes v llilctalremovnl (invu C iConventionnl N0..... No Yes Yes (Metal removal (invent. No No-... No.... No.... No..... No Yes-.. D 550 {Conventional No..... Yes-.- Yes Metnlreniovnl (invent)... No..... No.... No..... No-. No..... No-... Yes...- Yes.....

Using conventional pretreatments, the coating on copper substrates low in oxygen content (less than about 100 ppm) showed a tendency to blister and peel at temperatures above 1300 F., whereas electroplates on base metals having higher oxygen content blistered and peeled at significantly lower temperatures. Using the pretreatment in accord with the teachings of this invention, coatings deposited on low oxygen content substrates neither blistered nor peeled at temperatures near the melting point of the metal. Deposits plated on higher oxygen content substrates subjected to the instant pretreatment still showed some tendency to blistering and peeling, but not until temperatures are reached which are about 700 F. higher than those prepared by conventional methods of pretreatment.

in a second experiment, two sets of five panels each, were machined and cleaned as above. One set having a very low (25 ppm) bulk oxygen content and the other having a high (470 ppm) bulk oxygen content. The panels were then dissolved to various depths (reported in Table II, below), with the exception of one panel in each set receiving no further treatment (i.e., conventional pretreatment only). The panels were then plated as above and tested for blistering, by incrementally increasing the temperature in 100 F. stages, maintaining temperature for minutes, and noting the occurrence of blistering. From Table ll, it may therefore be seen, that where for a very low bulk oxygen substrate, only about 0.18 mils of metal removal is sufficient to provide adequate resistance to blistering up to temperatures of 1,400 F., a removal of 1.8 mils of the high oxygen substrate was still not adequate for providing resistance to blistering at l,400 F. But, even for these high oxygen substrates, the anodic dissolution treatment of this invention provided significantly enhanced resistance over that of conventional cleaning methods. Thus, if the bulk oxygen content of the base metal is not known, or if economics were not a primary concern, it may be seen that for substrates with bulk platiiig should be accomplished immediately the dissolution operation.

We claim:

1. A method for reducing blistering and peeling failures of plated, copper base articles, employed at service temperatures greater than 500 F.; in which, prior to plating, the surface of said article is dissolved to a depth sufficient to decrease the surface oxygen content to a level, approaching that of the base metal, wherein the depth of dissolution fora service temperature greaterthan 1,100 E, is at least 0.12 mils and is greater than that represented by the l,l00 F. line of FIG. 2, said depth at a desired service temperature increasing in proportion to the bulk oxygen content of the base metal.

2. The method of claim 1, wherein said base article has a bulk oxygen content greater than about ppm, and said depth of dissolution is greater than 0.2 mils.

3. The method of claim 1, wherein said plating is conducted to recondition said copper base article, and is accomplished by the electrodeposition of a layer consisting essentially of copper.

4. The method of claim 3, wherein the resistance to said failures is enhanced by employing a base article with a bulk oxygen content of less than 100 ppm.

' 5. The method of claim 3, wherein said base article has a bulk oxygen content less than about 600 ppm, and the enhancement of resistance to failure is insured by the dissolution of at least 1.0 mil of metal surface.

6. The method of claim 3, wherein said copper base article is a casting mold.

7. The method of claim 3, wherein said depth of dissolution for a service temperature greater than 1,300 F., is at least 0.18 mils and is greater than that represented by the l,300 F. line of FIG. 2.

8. The method of claim 7, wherein said base article has a bulk oxygen content greater than about 100 ppm, and said depth of dissolution is greater than 0.4 mils.

subsequent to 

2. The method of claim 1, wherein said base article has a bulk oxygen content greater than about 100 ppm, and said depth of dissolution is greater than 0.2 mils.
 3. The method of claim 1, wherein said plating is conducted to recondition said copper base article, and is accomplished by the electrodeposition of a layer consisting essentially of copper.
 4. The method of claim 3, wherein the resistance to said failures is enhanced by employing a base article with a bulk oxygen content of less than 100 ppm.
 5. The method of claim 3, wherein said base article has a bulk oxygen content less than about 600 ppm, and the enhancement of resistance to failure is insured by the dissolution of at least 1.0 mil of metal surface.
 6. The method of claim 3, wherein said copper base article is a casting mold.
 7. The method of claim 3, wherein said depth of dissolution for a service temperature greater than 1,300* F., is at least 0.18 mils and is greater than that represented by the 1,300* F. line of FIG.
 2. 8. The method of claim 7, wherein said base article has a bulk oxygen content greater than about 100 ppm, and said depth of dissolution is greater than 0.4 mils. 